Our microphones go down to about 10 Hz; this was well outside the sensitive band for iLIGO, but wont be for aLIGO, so we would like to be able to monitor lower acoustic frequencies. For this reason, the Eotvos group in Hungary has developed infrasound microphones. A prototype in S6 showed coherence between DARM and infrasound (here) suggesting that these sensors may be important. At the end of last week, Gabor, Peter, Zsolt, and I installed infrasound sensors, one in each of the three stations.

The figure shows the coherence between two of the infrasound microphones in a huddle, demonstrating that they detect infrasound with high SNR between about 3e-3 Hz and 1 Hz. Between 1 Hz and 50 Hz there are regions between peaks (the peaks are probably LVEA modes) where the SNR drops, reducing the coherence. But the infrasound range does extend high enough in frequency so that it detects in the same region as the regular microphone (note the green trace coherence between the regular microphone and the infrasound microphone between 10 and 50 Hz). The peak evident in the infrasound traces at about 38 Hz is likely the lowest resonance of the membrane. The infrasound microphones do not appear to be sensitive to vibration: the black trace shows that there is little coherence with the seismometer, except for several peaks between 20 and 30 Hz. We checked that these peaks (which come from clean rooms) were not coupling through the ground by seismically isolating one of the infrasound microphones. The clean room peaks remained in the signal from the isolated microphone, suggesting that signals from the fans were travelling through the air as well as the ground, as might be expected.

The microphones were calibrated by matching the traces to the trace from the LVEA mic in the overlap region. I calibrated the LVEA mic with a B&K 250 Hz pistonphone. The figure shows that the frequency response of the individual infrasound sensors is not completely flat (the traces diverge below the calibration frequency), so we need at least one more calibration point at low frequency to reduce the uncertainty.

Rough infrasound sensor calibration (calibrated at high f):

EY: 680 uPa/Count

LVEA: 680 uPa/Count

EX: 770 uPa/Count

CART: 910 uPa/Count

Gabor Szeifert, Péter Bojtos, Zsolt Frei, Robert Schofield